task<void> session(asio::ip::tcp::socket sock)
{
    try
    {
        char buf[1024];
        std::cout << "connected: " << sock.remote_endpoint() << std::endl;
        for ( ; ; )
        {
            act::error_code ec;
            auto len = co_await act::read_some(sock, asio::buffer(buf), ec);
            if (ec == asio::error::eof)
                co_return;
            co_await act::write(sock, asio::buffer(buf, len));
        }
    }
    catch (std::exception& e)
    {
        std::cout << "error: " << sock.remote_endpoint() << ": " << e.what() << std::endl;
    }
}

task<void> server(asio::io_service& io, unsigned short port)
{
    asio::ip::tcp::endpoint endpoint{asio::ip::tcp::v4(), port};
    asio::ip::tcp::acceptor acceptor{io, endpoint};
    asio::ip::tcp::socket sock{io};
    std::cout << "server running at: " << endpoint << std::endl;
    for ( ; ; )
    {
        co_await act::accept(acceptor, sock);
        session(std::move(sock));
    }
}

int main(int argc, char *argv[])
{
    asio::io_service io;
    server(io, std::atoi(argv[1]));
    io.run();

    return EXIT_SUCCESS;
}